1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to outcouplers for master oscillator power amplifier (MOPA) systems.
2. Description of the Related Art
The High Energy Laser (HEL), because of its rapid time of flight, pointing agility, precision, lack of collateral damage effects, and lack of traceable residue, is an effective weapon against a broad range of military targets. The diode-pumped solid-state laser, because of its high electrical efficiency, relatively low weight, compact packaging, lack of consumables (except sunlight or fuel), and lack of toxic and corrosive effluents is compatible with many military platforms, including fixed installations, ground vehicles, surface ships, submarines, rotocraft, tactical and strategic aircraft, and spacecraft.
One of the most attractive approaches for a continuous operation weapon-class, high brightness solid-state laser uses Yb:YAG slabs in a two-pass master oscillator/power amplifier (MOPA) configuration with a vector loop phase conjugate mirror (LPCM). The basic phase conjugate (PC) MOPA architecture uses a small master oscillator, which delivers a low-power single-mode reference beam through an optical input/output coupler element (outcoupler) to the output end of a high power amplifier beamline. The beam is then amplified to medium power, picking up thermal lensing and wedging aberrations and is depolarized due to thermal stress birefringence. At this point the beam enters a phase conjugate mirror, which reverses the wavefront of the beam. The reflected, phase conjugate beam then makes a return pass through the aberrated amplifier beamline and the original wavefront is restored. A high power, high beam quality beam is delivered via the outcoupler.
One of the most critical components in this PC MOPA laser architecture is the outcoupler, which is responsible for inserting the low power master oscillator beam into the amplifier beamline and extracting the amplified beam from the beamline in a separate path. Ideally, the outcoupler would insert the oscillator beam with zero loss, extract the amplified beam with zero feedback into the oscillator, and generate no distortions that cannot be corrected by the LPCM. Several outcoupler schemes have been developed and used with the PC MOPA architecture. The Scalable High Energy Raman Laser (SHERL) was the first moderate power PC MOPA device demonstrated in the U.S., and used a Brewster plate in conjunction with a quarter wave plate for polarization outcoupling. This scheme is disclosed by Hans W. Bruesselbach in U.S. Pat. No. 4,734,911, entitled “Efficient Phase Conjugate Laser,” issued Mar. 29, 1988 (the teachings of which are incorporated herein by reference). This approach provided very efficient transmission of the amplified beam with low oscillator feedback. However, it was not efficient in the injection of the oscillator beam into the amplifier beamline. Therefore, a higher power oscillator is required than would be required with an ideal outcoupler.
The most straightforward outcoupler approaches for high power are based on reciprocal optical elements such as reflective/refractive beamsplitters and diffraction gratings. These devices are designed to promote efficient outcoupling for the high power beam. The coupling efficiency of the master oscillator input path, however, may be very low for these devices, necessitating a relatively high power master oscillator. High oscillator power is problematic for two reasons: (1) reduced overall efficiency of the MOPA and (2) difficulty in obtaining high oscillator beam quality.
Lower power PC MOPA systems utilized a polarizing beamsplitter in conjunction with a permanent-magnet Faraday rotator and quartz rotator combination to provide a non-reciprocal optical path for efficient outcoupling. The Faraday rotator and polarization beamsplitter approach works well at average powers up to a kilowatt. The HEL application, however, calls for hundreds of kilowatts to megawatts of average power, which is beyond the current state-of-the-art in Faraday devices.
Non-Faraday outcoupler techniques based on non-reciprocal interferometric elements have been proposed which show promise in scaling to weapon-class power levels. In the early 1990s, several high average power interferometric outcoupler configurations were developed which rely on the Stokes frequency shift inherent in the stimulated Brillouin scattering (SBS) phase conjugation process to create a non-reciprocal optical path. The first disclosed by T. O'Meara in U.S. Pat. No. 5,126,876, entitled “Master Oscillator Power Amplifier with Interference Isolated Oscillator,” issued Jun. 30, 1992, the teachings of which are incorporated herein by reference, uses a Mach-Zender interferometer as the outcoupling element directly. This interferometer is used as the non-reciprocal element to separate the input and output paths through constructive interference in one direction and destructive interference in the other. Because the Stokes shift is fixed by the material parameters of the SBS medium (determined by sound velocity), the wavelength of the master oscillator and the length of the interferometer legs must be controlled to ensure good master oscillator isolation and input/output coupling efficiency.
The second interferometric approach uses the interferometer in the phase conjugate leg to effect a 90 degree polarization rotation on the output pass, which creates a non-reciprocal path through a polarization beamsplitter. The operation of this interferometric polarization outcoupler is disclosed in Basov et al, “Laser Interferometer with Wavelength-Reversing Mirrors,” Sov. Phys. JTEP, Vol. 52, No. 5, November 1980, pp 847–851. Inventive improvements to this basic scheme were disclosed by D. Rockwell in U.S. Pat. No. 5,483,342, entitled “Polarization Rotation with Frequency Shifting Phase Conjugate Mirror and Simplified Interferometric Output Coupler,” issued Jan. 9, 1996.
A problem with these prior art interferometric outcoupler approaches is that they must be used with a PCM that which has a fixed and predetermined frequency shift, typically an SBS PCM. The SBS PCM has several disadvantages: it does not work well with continuous waveforms, and it requires high peak power but cannot handle high average power. Furthermore, the prior art interferometric outcoupler approaches are sensitive to length changes in the interferometer optical paths resulting from thermal expansion and warping of the structure, plastic deformation and creep, shock and vibration induced structural compliance, or refractive index changes of the optics and intervening atmosphere, as well as any changes in the frequency of operation of the oscillator or phase conjugate mirror.
Hence, a need exists in the art for an efficient outcoupler for high power MOPA systems which can compensate for any frequency changes in the outcoupler, oscillator, and phase conjugate mirror.